The present invention relates to the stimulation of very low permeability formations.
Hydraulic fracturing of very low permeability formations, also known as tight formations (including tight gas formations), such as the Barnett, Woodford, or Fayetteville shale formations, is common. At present it is estimated that there are hundreds of rigs operating in the Barnett shale alone. Wells are drilled horizontally to access the tight formations and production is then stimulated by one or usually a plurality of fracture treatments.
The wells are drilled with various types of drilling fluids. Residual drilling fluid present upon the completion of drilling is approximately 240 m3 at each rig (1500 bbl/rig). When the well is complete the residual drilling fluids must be disposed of at relatively high cost per unit volume (m3 or bbl).
In the mid 1990's, most of the tight gas reservoirs were fractured utilizing crosslinked gelled fluids. In an effort to reduce treatment costs, slick water fracturing has emerged as the method of choice.
The fracturing treatments of tight formations are characterized by extended fracture closure times where the fracture may remain open for hours after injection ceases. The tight formations of interest have permeability on the order of several hundred nanodarcy, so fluid-induced damage to the fracture face from leakoff of fracturing fluids is not always a concern. These shale formations have such low permeability that the wells can be effectively stimulated with only a minimal final fracture conductivity, on the order of 0.3 to 3 mD-m (1 to 10 mD-ft). Consequently fracturing fluid typically contains no or only small quantities of solids, e.g., 100 mesh sand, 40/70 mesh sand, or 30/50 mesh sand as proppant, and has low viscosity, e.g., slick water, which can also facilitate a reduced fracture height growth because of the lower viscosity.
Whereas fracture treatments on moderate or highly conductive formations can involve 950 m3 (6000 bbl) of fluid and 45-90 metric tons (100,000-200,000 pounds) of proppant; in a typical tight gas treatment, thousands of cubic meters (several million gallons) of water are pumped at a typical rate of 10 m3/minute (65 bbl/min (bpm)) with sand ranging in concentration from 0.03 g/mL to 0.12 g/mL (0.25 to 1.0 lb/gal (ppg)), and other additives including scale inhibitor, friction reducers, biocides, clay swelling inhibitors, oxygen scavengers, surfactants and the like.
The published data show that 60 to 90% of the injected fluids commonly stay in the tight gas reservoirs after the stimulated well is placed in production. It is likely that large quantities are trapped in the area surrounding the fracture and within the fracture itself.
The statements in the preceding section merely provide background information related to the present disclosure and may not constitute prior art.